Websites typically incorporate both original content and third party content into the pages (or other data) that they serve to visitors. For example, when a visitor accesses a website that serves news articles, the text of the article may be served by the news website, while images may be served by a third party image hosting site. If the news website is compromised, visitors' client devices may be exposed to unauthorized (and frequently malicious) programs/modifications (hereinafter collectively “malware”). Unfortunately, even if the news website is itself secured, visitors may nonetheless be exposed to malware if the third party image hosting site is compromised.